Limiting nox emissions

ABSTRACT

A method for controlling an internal combustion engine limits emission of undesirable compounds of nitrogen and oxygen and provides increased transient power.

RELATED APPLICATIONS

This application claims priority to U.S. application Ser. No.61,540,073, having a filing date of Sep. 28, 2011, and U.S. applicationSer. No. 61/557,077 having a filing date of Nov. 8, 2011, thedisclosures of which are incorporated herein by reference in theirentirety.

FIELD

This disclosure relates to operation of a compression ignition internalcombustion engine and exhaust aftertreatment to limit emission ofnitrogen oxides. Embodiments relate to transient increased poweroperation of the engine.

BACKGROUND

Internal combustion engines are widely used to power electricgenerators, vehicle engines, and the like. Emissions from internalcombustion engines and their impact on the environment are of increasingconcern. Because of this concern, regulations that limit the amount ofvarious emission gases that may be exhausted to the environment fromthese engines have been adopted. Specific attention has been directed toreducing emissions of nitrogen and oxygen compounds, including NO andN₂O, that are collectively referred to as NO_(x). These compounds areformed by in the combustion chambers of internal combustion engines whennitrogen and oxygen reach a high temperature due to combustion of fuel.

Three way catalysts (referred to as TWCs) have been widely used toreduce emissions of NO_(x), from spark ignition engines. TWCs becomeineffective in reducing NO_(x) in exhaust when oxygen is present. Thisis not a significant limitation for use with spark ignition engines thatoperate with an air/fuel mixture near a stoichiometric point and createexhaust that has little oxygen. However, TWCs experience reducedperformance as they age, even with robust catalyst materials.

Limiting undesirable emissions from compression ignition engines remainsa goal of design and control of compression ignition engines and theirexhaust aftertreatment systems. Compression ignition engines arecommonly operated with much more air entering the engine than isrequired for combustion of the amount of fuel provided. Such mixtures ofair and fuel are referred to as “lean” while operation with less air isreferred to as a “rich” mixture. Combustion of lean air and fuelmixtures creates high temperatures within engine combustion chambers.The lean mixture includes significant amounts of oxygen that is notconsumed in combustion and is available to combine with nitrogen to formNO_(x). The lean air/fuel mixture creates exhaust having a significantamount of oxygen that renders TWCs ineffective in reducing NO_(x).

Different strategies for limiting emission of NO_(x) by compressionignition engines are known. One approach is to provide for selectivecatalytic reduction of the nitrogen and oxygen compounds in engineexhaust. Another approach is to prevent formation of NO_(x) by returningengine exhaust gas to a combustion chamber of the engine, referred to as“exhaust gas recirculation” (EGR). Introducing exhaust gas into theengine combustion chamber reduces creation of NO_(x) in two ways.Exhaust gas displaces air and thereby reduces the amount of oxygenavailable for creation of NO_(x). Exhaust gas also functions as adiluent that is heated and thereby results in lower combustiontemperature in the combustion chamber.

Compression ignition engines that operate with significant exhaust gasrecirculation may not require aftertreatment to reduce NO_(x) emission.Operating a compression ignition engine under these conditions reducesthe power created by the engine due to the limited amount of fuel thatis combusted during such operation.

When greater power is required from a compression ignition engine,either at constant engine speed or more commonly during operation toaccelerate a vehicle, more fuel must be combusted to provide therequired power. Typically, higher power is required to be produced bythe engine within a short period of time. The requirement that power beincreased within a short response time requires both more fuel anddifferent fueling timing than is required for prolonged low poweroperation.

Compression ignition engines are commonly controlled by engine controlunits (ECU) that monitor conditions of engine operation and that operateactuators that control engine operation. Conditions that are monitoredinclude mass air flow into the engine, intake manifold temperature andpressure and engine speed. Oxygen content of exhaust and/or intake flowmay also be monitored. Fuel injection may introduce fuel into the enginefor combustion. Fuel injectors may permit control of the amount, timingand pattern of injection of fuel into the engine. Compression ignitionengines may also include a turbocharger that has a variable nozzlemechanism that may be controlled to control the compression (boost) ofair that is provided to the engine from the turbocharger. Compressionignition engines may also have an EGR valve that controls the amount ofexhaust gas that is diverted into the air stream to the engine. An ECUcan control fuel injection, turbocharger variable nozzle mechanisms andEGR valves for engines that include these controllable devices. Thecapability of ECUs to monitor and control operation of compressionignition engines provides the capability to change operation of theengine.

ECUs may control engine operation by combinations of set points forengine conditions and control actuators. An ECU may acquire informationfrom sensors and determine actuator settings based on that sensorinformation and operation set points. ECUs may be programmed to controlengine operation based on either stored settings or calculate settingsbased on sensor inputs.

BRIEF SUMMARY

In one aspect of the present technology, operation of a compressionignition engine with an exhaust aftertreatment system is described thatpermits the engine to provide quickly increased power and maintainsemissions of nitrogen and oxygen compounds at a desired level.

An additional described aspect resides in providing an aftertreatmentsystem for an internal combustion engine with a capacity to eliminateundesirable compounds from an exhaust gas stream during transientoperation to increase power.

Another described aspect of the present technology relates tocontrolling an internal combustion engine to limit emission ofundesirable compounds from the engine and controlling fueling of theengine during transient operation to increase power so that the exhauststream has a composition for which aftertreatment is effective.

Yet another aspect of the present technology resides in providing anaftertreatment system for a compression ignition engine that includes acatalyst coated surface at which catalyzed reactions reduce the amountof NO_(x) in exhaust during operation of the engine to provide highpower.

Still another aspect of the present technology provides an exhaustaftertreatment system that includes a three-way catalyst for reducingthe amount of NO_(x) in exhaust from a compression ignition engine.

Another described aspect of the present technology relates to an exhaustsystem that modifies the composition of exhaust gas to assure reductionof NO_(x) by a three way catalyst.

It is another aspect of the present technology to provide an exhaustsystem for a compression ignition engine that has an active hydrocarboninjector configured to inject hydrocarbon into an exhaust stream toenhance NOx reduction over a three way catalyst converter.

Still another aspect of the present technology relates to methodswherein a determination is made as to whether a compression ignitionengine is fueled by a rich air/fuel mixture, and injecting hydrocarboninto the exhaust gas before exhaust enters a three way catalyst when arich mixture is detected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic representation of a described compressionignition engine with exhaust aftertreatment.

FIG. 2 shows the relationship between the amount of exhaust gas in theintake mixture and the amount of nitrogen compounds in engine exhaust.

FIG. 3 shows an embodiment of an aftertreatment system.

FIG. 4 shows the amount of NO_(x) emitted by an aftertreatment systemfor increasing power operation of a compression ignition engine at leanair/fuel mixture operation and at near stoichiometric air/fuel mixtureoperation.

FIG. 5 illustrates a schematic representation of a hydrocarbon injectorsystem for an exhaust system.

FIG. 6 is a flow diagram of operations that may be executed to injecthydrocarbon into an exhaust gas of a compression ignition engine.

FIG. 7 show variations in speed and torque during transient operation ofan exemplary diesel engine.

FIG. 8 show variations in NO_(x) output from a three-way catalystconverter under the transient operations shown in FIG. 7, wheredifferent amounts of hydrocarbon are injected into the exhaust gasstream.

DESCRIPTION OF EMBODIMENTS

Embodiments described herein concern control of a compression ignitionengine and aftertreatment of exhaust created by the engine. Inparticular, embodiments concern control of a compression ignition engineto limit emission of NO_(x) when the engine operates to produce power ator near its capacity. In addition, embodiments concern improvingtransient response to requests for increased power by a compressionignition engine that has exhaust gas recirculation and limiting emissionof nitrogen and oxygen compounds.

Embodiments will be described more fully hereinafter with reference tothe accompanying drawings, in which embodiments are shown. Likereference numbers refer to like elements throughout. Other embodimentsmay, however, be in many different forms and are not limited to theembodiments set forth herein. Rather, these embodiments are examples.Rights based on this disclosure have the full scope indicated by theclaims.

FIG. 1 shows a schematic depiction of a compression ignition engine 10and exhaust aftertreatment apparatus 40. Operation of the compressionignition engine 10 is monitored and controlled by an ECU 50 as describedbelow.

Air enters the engine 10 at air inlet 24. A mass air flow sensor 21senses the amount of air entering the engine through air inlet 24. Airis directed from air inlet, 24 to the low pressure turbochargercompressor 22 which compresses the air. Compressed air is directed fromthe low pressure turbocharger compressor 22 to the high pressureturbocharger compressor 18 which further compresses the air. Compressedair is then directed to an intake manifold 16. As further describedbelow, an exhaust gas recirculation system 28 selectively directsexhaust gas into the compressed air entering the intake manifold 16.

Air and, under some operating conditions, exhaust enters the cylinders12 of the engine 10 through the intake manifold 16. An intake flowtemperature sensor 19 and an intake flow pressure sensor 17 are mountedto the intake manifold 16 to measure the temperature and pressure of theflow entering the cylinders 12 of the engine 10. An injector 14 isprovided for each cylinder 12 to inject fuel into the cylinder 12.

After combustion of fuel in the cylinders 12, exhaust from the cylinders12 is directed to an exhaust manifold 26. The exhaust manifold 26directs exhaust to a connection to the exhaust recirculation system 28and to the high pressure turbocharger turbine 36. An exhaust oxygensensor 23 measures the amount of oxygen in the exhaust leaving theengine 10. Oxygen sensor 23 may be a lambda sensor.

The exhaust gas recirculation system 28 provides a passage for exhaustleaving the exhaust manifold 26 to enter the flow of compressed air fromthe turbocharger compressor 18 entering the intake manifold 16. Theexhaust entering the exhaust recirculation system 28 is directed to acontrollable EGR valve 34 and then to an exhaust cooler 32 that lowersthe temperature of exhaust. Exhaust is then directed into the flow ofcompressed air from the turbocharger compressor 18. The pressure ofexhaust in the exhaust manifold 26 is higher than pressure in the intakemanifold 16 thereby causing exhaust to flow from the exhaust manifold26, through the exhaust gas recirculation system 28, and into the intakemanifold 16.

Exhaust that does not flow through the exhaust recirculation system 28flows to and through the high pressure turbocharger turbine 36. The highpressure turbocharger turbine 36 is driven by exhaust from the exhaustmanifold 26 and drives the high pressure turbocharger compressor 18. Thehigh pressure turbocharger turbine 36 includes a controllable variablenozzle. Opening that controllable variable nozzle decreases driving ofthe high pressure turbocharger turbine 36 and consequently decreasecompression of air by the high pressure turbocharger compressor 18.Opening the variable nozzle of the high pressure turbocharger turbine 36also decreases resistance of the high pressure turbocharger turbine 36to flow of exhaust, thereby lowering pressure of exhaust within theexhaust manifold 26 and exhaust gas recirculation system 28. Conversely,closing the variable nozzle of the high pressure turbocharger turbine 36increases pressure of exhaust within the exhaust manifold 26, increasesdriving of the high pressure turbocharger turbine 36, and increasescompression of air by the high pressure turbocharger compressor 18.

Exhaust is directed from the high pressure turbocharger turbine 36 to alow pressure turbocharger turbine 38 that drives the low pressurecompressor 22. Exhaust is directed from the low pressure turbochargerturbine 38 to an inlet 42 of the exhaust aftertreatment system 40.

The aftertreatment system 40 is configured to reduce the amount ofundesirable components of exhaust. As further described below, theconfiguration and operation of the engine 10 creates exhaust havingcharacteristics that are the basis for the configuration of theaftertreatment system 40. Exhaust that has been subjected to treatmentby the aftertreatment system 40 exits the aftertreatment system 40 atexit 44 from which it is directed to an exhaust discharge outlet 46.

The ECU 50 controls operation of the engine 10 based on measurementsprovided by engine sensors. The intake mixture pressure sensor 17,intake mixture temperature sensor 19, mass air flow sensor 21, exhaustoxygen sensor 23 and an engine speed sensor 25 are connected tocommunicate measurements to the engine control unit 50 as indicated inFIG. 1. The ECU 50 controls each of the injectors 14 to control thetiming and amount of fuel that is injected into the cylinder 12. The ECU50 also controls the controllable EGR valve 34 to open and close thevalve 34 thereby increasing and decreasing the flow of exhaust gas intothe intake manifold 16. The ECU 50 also controls the variable nozzle ofthe high pressure turbocharger turbine to increase and decreasecompression of air by the high pressure turbocharger compressor 18.

Conventional operation of a compression ignition engine provides moreair to the engine than is required for combustion of the fuel provided.Under low load conditions, the air to fuel ratio can be in the range of50:1 to 100:1. Under such conditions, displacing an amount of air bymixing exhaust gas with air entering the engine does not unacceptablyreduce the power produced by the engine. Under such low load conditions,the exhaust gas recirculation system 28 of the compression ignitionengine 10 can reduce the formation of nitrogen and oxygen compounds bythe engine 10. Because exhaust gas includes a significant amount ofinert diluent and because the exhaust gas is cooled before combiningwith air entering the engine 10, the presence of exhaust gas in thecylinder 12 reduces the temperature in the cylinders 12 due tocombustion and thereby reduces the creation of nitrogen and oxygencompounds. FIG. 2 shows the relationship between the amount of engineexhaust in the intake mixture and the amount of nitrogen and oxygencompounds in the engine exhaust for low load operation of the engine.

Operation of the exhaust recirculation system 28 to introduce cooledexhaust gas into the intake stream into the engine 10 both reduces theformation of NO_(x) and reduces the power produced by the engine 10. Thepower created by the engine 10 is reduced because the amount of air thatis available for supporting combustion is reduced by the amount of theinert components of the exhaust gas. This is not a significantdisadvantage when high power is not required from the engine. Forexample, an internal combustion engine that powers a vehicle is notrequired to produce power at or near its capacity for long periods oftime, such as when the vehicle maintains a constant speed on a surfacethat offers little resistance to movement of the vehicle. Such cruisingmay only require engine power in the range of one half to three quartersthe fully fueled engine power. Combining a lean mixture and exhaust gasrecirculation creates an operating condition in which both fuel economyand low emission of nitrogen and oxygen compounds are achieved.

When power near the capacity of a compression ignition engine isrequired, such as to accelerate a vehicle, operation of the enginechanges significantly. Internal combustion engines convert energy fromcombustion of fuel into mechanical energy. An increase in mechanicalpower from an internal combustion engine requires an increase in theamount of fuel consumed by combustion in the engine. To increase powerof a compression ignition engine to near its capacity, the amount offuel provided to the cylinders must be increased to the capacity of theengine to combust the fuel. That capacity depends on the amount of airavailable for combustion. To increase the power of a compressionignition engine, both the amount of air and the amount of fuel providedto the engine must be increased. For acceleration of a vehicle,increased power is typically required within a short response periodafter power is requested of the engine.

Under conditions for which a compression ignition engine is required toproduce power near its capacity, such as to accelerate a vehicle,introducing exhaust into the stream of air entering the engine is asignificant disadvantage. To the extent that the exhaust displaces airfrom the flow entering the cylinders 12, the exhaust lowers the capacityof the engine to consume fuel. Further, diverting exhaust to the intakestream reduces energy that is available to drive turbochargers.Referring again to FIG. 1, air is provided to cylinders 12 of the engine10 by the high pressure turbocharger compressor 18 which received airfrom the low pressure turbocharger compressor 22. The exhaust that itdiverted to the intake flow is not available to drive the turbochargerturbines 36 and 38 thereby lowering their capacity to drive thecompressors 18 and 22, respectively, and lowering the compression of airforced into the cylinders 12. Under conditions such as transientincreased power, exhaust gas recirculation cannot be relied on to limitemissions of NO_(x).

Power of the compression ignition engine 10 is increased by increasingthe amount of air entering the engine and providing as much fuel as canbe consumed combusted by the available air. Fueling the engine by theinjectors 14 is a straightforward control available from the ECU 50.Increasing the amount of air provided to the cylinders 12 is lessdirect. Air is provided to the intake manifold 16 by the turbochargercompressors 18 and 22, which are driven by the turbines 36 and 38,respectively. Two controls are available to increase the power producedby the high pressure turbocharger turbine 36. Closing the controllableEGR valve 34 increases the amount of exhaust that is available to drivethe high pressure turbocharger turbine 36. In addition, closing thecontrollable variable nozzle of the high pressure turbocharger turbine36 increases the pressure of exhaust driving the high pressureturbocharger turbine 36 and increases the power produced by thatturbine.

When power is demanded of the engine 10 that is at or near its capacity,the ECU 50 invokes a fueling strategy that provides an air fuel mixtureto the cylinders 12 that is at or richer than the stoichiometric ratio.Operating the engine 10 with a richer than stoichiometric air/fuelmixture and without exhaust gas recirculation creates an exhaust streamthat has a large amount of NO_(x) and is also low in oxygen and high inhydrocarbons and carbon monoxide. The fueling pattern, that is thetiming and duration of introduction of fuel may be changed as the amountof fuel provided is increased or decreased. Fueling for a lean air/fuelmixture may be by providing a pilot injection and a main injection offuel. Richer mixtures can increase production of soot by a compressionignition engines. The amount of soot that is produced during fueling ofrich mixtures may be reduced by fuel injection pattern and timing asdisclosed by patent application entitled “Fuel Injection Pattern andTiming” filed of even date herewith.

FIG. 3 shows an aftertreatment system 40 that is configured to maintainacceptable emissions treating exhaust from the engine 10. Exhaust entersthe inlet 42 and is directed to through a diesel oxidation catalyst 52.The diesel oxidation catalyst 52 is formulated to reduce the amounts ofcarbon monoxide, hydro carbons, soluble organic fraction, andpolynuclear aromatic hydrocarbons that are present in exhaust from theengine 10 during operation with a lean air fuel mixture and exhaust gasrecirculation.

After passing through the diesel oxidation catalyst 52, exhaust passesthrough a three way catalyst 54. The three way catalyst 54 is formulatedto reduce hydrocarbons, carbon monoxide, and NO_(x). The three waycatalyst 54 functions for exhaust having low oxygen content such as theexhaust created by fueling the engine 10 to about or richer than astoichiometric air to fuel ratio. This fueling creates exhaust havingincreased amount of carbon monoxide (CO). By the nature of its reductantproperties, CO reduces the NO_(x) as the exhaust passes through thethree way catalyst in accordance with the following reactions:

NO+CO→½N₂+CO₂

2NO+CO→N₂O+CO₂

In this way, the three way catalyst reduces emissions of NO_(x) for richfueling conditions when EGR system 28 is not active to reduce creationof NO_(x).

During operation of the engine 10 with a lean air/fuel mixture, the EGRsystem 28 reduces formation of NO_(x) thereby keeping the level ofNO_(x) in exhaust entering the aftertreatment system 40 low. Whenfueling of the engine 10 is increased to increase power created by theengine 10, exhaust recirculation is reduced or stopped as describedabove. As a result, the levels of NO_(x) and CO in the exhaust enteringthe aftertreatment system 40 increase. As the air/fuel ratio provided tothe cylinder 12 reaches stoichiometry, the amount of oxygen decreasesand the three way catalyst 54 becomes active to reduce the level ofNO_(x) in exhaust passing through the aftertreatment system 40. Thethree way catalyst thereby provides just in time reduction of NO_(x) inexhaust passing through the aftertreatment system 40 during transientoperation of the engine 10 to increase power.

Finally, after passing through the three way catalyst 54, exhaust passesthrough the diesel particulate filter 56. The diesel particulate filter56 captures particulate matter in the exhaust. It will be appreciatedthat the diesel oxidation catalyst 52, three way catalyst 54 and dieselparticulate filter 56 may be combined as a single unit, as depicted byFIG. 3, may be separate components or may be otherwise combined. Theorder in which exhaust passes through the diesel oxidation catalyst 52,three way catalyst 54 and diesel particulate filter 56 may also be otherthan as depicted.

FIG. 4 shows levels of NO_(x) in exhaust leaving the outlet 44 of theaftertreatment system 40 for operation of the engine 10 with a constantpercent of EGR provided to the engine while the torque produced by theengine is increased. Levels of NO_(x) emissions for two fuel mixturesare shown by FIG. 4, one for engine operation at a lean air/fuel mixtureand one for engine operation at an approximately stoichiometric air/fuelmixture. The lower graph of FIG. 4 shows a torque demand curve 101, acurve 103 that shows the torque produced by the engine at operation witha lean air/fuel mixture, and a curve 105 that shows the torque producedby the engine at operation at an approximately stoichiometric air/fuelmixture. Those torque curves show that approximately the same torqueresponse can be achieved by the approximately stoichiometric mixture asthe lean mixture.

The amount of NO_(x) emitted for those two air/fuel mixture during atime that includes increasing torque is shown by the upper curves ofFIG. 4. Curve 113 is the level of NO_(x) emissions for lean air/fuelmixture operation, and curve 115 is the level of NO_(x) emissions forapproximately stoichiometric air/fuel mixture operation. Those emissionscurves demonstrate the effectiveness of the aftertreatment with a TWC tosubstantially prevent increased emission NO_(x) during transientoperation of the engine to increase engine torque.

Fueling a compression ignition engine with a rich air/fuel mixtureproduces less CO than is produced by a comparable spark ignitiongasoline engine. Therefore, the amount of NO_(x) that is reduced by COin exhaust of a compression ignition engine is limited. Reduction ofNO_(x) by the TWC may be increased by introducing hydrocarbons (HC) intothe exhaust of a compression ignition engine upstream from the TWC. Whenhydrocarbon (HC) is injected into the exhaust stream, it reacts with theNO_(x) in the exhaust stream as the exhaust stream flows over the TWC 35in accordance with the following equation, with the byproducts beingnitrogen, carbon dioxide, and water:

HC+NO→N₂+CO₂+H₂O

The hydrocarbon level of exhaust may be increased by the ECU 50 causingthe injectors 14 to inject fuel into a cylinder 12 during a postinjection portion of the fuel injection cycle. In addition to lateintroduction of hydrocarbon into exhaust by injectors 14 or as analternative to that fuel injector introduction, hydrocarbon may beinjected into exhaust at a location in the exhaust system that isupstream of the TWC 54. FIG. 5 is a schematic illustration of an activehydrocarbon injector system 80 and a section 85 of an exhaust systemthat includes the aftertreatment system 40 and an adjacent portion of anexhaust system that is upstream from the aftertreatment system 40. Theexhaust system section 85 includes a burner 82 and a doser 84 upstreamfrom the aftertreatment system 40. The active hydrocarbon injectorsystem 80 includes a hydrocarbon injector 86 that communicates with theECU 50.

As described above, the ECU 50 receives information from a plurality ofsensors that indicate whether the diesel engine 10 is being fueled by arich mixture. The sensors may include the sensors described above andadditionally one or more sensors in the doser 84, one or more sensors inthe burner 82; and/or one or more sensors in the aftertreatment system40. Sensors may also be placed elsewhere in the diesel engine 10provided they may be used by the ECU 50 to determine whether the dieselengine 10 is being fueled by a rich air/fuel mixture.

The types of sensors that may be used, as well as their placement, aredependent on system design parameters. For example, the oxygen sensor 23may be used to determine the amount of oxygen in the exhaust gas streamexiting the diesel engine core. Fuel sensors also may be used todetermine the amount of fuel in the exhaust gas stream. Fuel injectionparameters used by the ECU 50 to control operation of the diesel engine10 are also useful in determining whether the diesel engine 10 is beingfueled by a rich mixture. Other sensor types and configurations may alsobe employed.

The active hydrocarbon injector system 80 may include a hydrocarboninjector 86 that is connected for control by the ECU 50. The ECU 50 isconfigured to control the hydrocarbon injector 86 to inject ahydrocarbon into the exhaust system 85 when the information providedfrom the plurality of sensors indicates that the compression ignitionengine 10 is being fueled by a rich air/fuel mixture. Alternatively, theECU 50 may be configured to control the hydrocarbon injector 86 toinject hydrocarbon into the exhaust system 85 when reduction of NO_(x)by the three way catalyst 54 is desired or required, for example such aswhen exhaust gas recirculation is reduced or stopped. The ECU 50 maycontrol the operation of other components of the exhaust system section85, or a dedicated controller for the hydrocarbon injector system 80 maybe provided. The injected hydrocarbon may be in liquid or gaseous formand may be injected by the hydrocarbon injector 86 at one or morelocations.

According to certain embodiments, the hydrocarbon being injected may bediesel fuel, which may be generally supplied to the active hydrocarboninjector system 80 from the fuel tank or other reservoir of theassociated vehicle. Additionally, the ECU 50 may be configured tocontrol the quantity and/or duration of time that hydrocarbon isinjected into the exhaust stream may be adjusted, such as, for example,based on the degree to which the engine 10 is fueled by a lean or richair/fuel mixture. The ECU 50 may also be configured to account for avariety of other factors beside the condition of the exhaust stream whendetermining the quantity or duration for which hydrocarbons are to beinjected into the exhaust stream. For example, ECU 50 may furtherelevate the quantity of and/or duration that hydrocarbons are to beinjected into the exhaust stream based on the sensed and/or calculatedloss of performance of the TWC 54 such as performance loss ordeterioration associated with the aging of the TWC 54.

FIG. 6 is a flow diagram of operations that may be executed to injecthydrocarbon into exhaust gas of a compression ignition engine, such asengine 10. As shown, a flowing exhaust gas from the engine is providedat 205. The exhaust gas from the engine includes NO_(R). At operation210, a direct and/or indirect determination is made as to whether theengine 10 is being fueled by a lean or rich air/fuel mixture. If theengine 10 is not being fueled by a rich air/fuel mixture as decided atoperation 215, the method returns to operation 210. However, fueling bya rich mixture is detected at operation 215, an amount of hydrocarbon(HC) is injected into the gas exhaust which, in turn, is provided to theTWC at operation 220. At operation 225, the method may determine whetherthe injected hydrocarbon is sufficient to reach desired exhaustparameters. Such parameters may include whether the NO_(x) has beenreduced below a predetermined level by the TWC. If it has not, operation220 may be re-executed until the desired exhaust parameters are detectedat operation 225.

FIG. 7 shows variations in engine speed and outputted torque duringtransient operation of an exemplary diesel engine system, such as theone shown in FIG. 1. FIG. 8 show variations in NO_(x) output from theTWC under the transient operations shown in FIG. 7. Moreover, forexemplary purposes, FIG. 8 illustrates the effect of differenthydrocarbon injection rates per engine stroke have on the level ofNO_(x) that is outputted from the TWC. Moreover, FIG. 8 generallyidentifies the injection rates as “a,” “b,” “c,” “d,” and “e”, with therate of injection increasing in ascending order from “a” (lowestinjection rate) to “e” (highest injection rate). For comparisonpurposes, also charted is the level of NO_(x) entering into the TWC.Additionally, for further comparison purposes, in the absence ofinjecting hydrocarbons into the exhaust stream, the level of NO_(x)exiting the TWC would assumed to be the same as the level of NO_(x)entering the TWC.

The transient operations shown in FIG. 7 were executed multiple times toarrive at the data shown in FIG. 8. In each execution, different amountsof hydrocarbon were injected into the exhaust gas stream. As shown byFIG. 8, among other things, the overall NO_(x) that is outputted fromthe TWC during such transient conditions is reduced when hydrocarbon hasbeen injected into the exhaust gas stream. As shown, various levels ofhydrocarbon injection achieve different levels of NO_(x) reduction inthe exhaust gas outputted from the TWC. Optional amounts of hydrocarboninjection for various systems may be derived by employing controlledtests on the engine on a dynamometer.

The present invention is not limited to use with any specific controlscheme. The invention can be adapted to a variety of internal combustionengines.

1. A method of operating a compression ignition engine to limit exhaustemissions during transient operation to increase engine powercomprising: operating the engine at a first power output by providing afirst air/fuel mixture to a combustion chamber of the engine; receivinga request for an increased power output from the engine, that is greaterthan the first power output; providing a second air/fuel mixture to thecombustion chamber of the engine, the second air/fuel mixture comprisinga greater amount of fuel than the first air/fuel mixture; and treating aflow of exhaust gas with a non-urea aftertreatment to reduce a level ofpollutant in the exhaust gas during a fuel enriched operation of theengine to invoke the real-time removal reduction of the pollutant whileincreasing engine power.
 2. The method of claim 1, wherein the pollutantis NOx.
 3. The method of claim 2, wherein the NOx is reduced to lessthan 0.2 gram per horsepower-hour work performed by the engine.
 4. Themethod of claim 2, wherein the NOx is reduced to less than 0.3 gram perhorsepower-hour work performed by the engine.
 5. The method of claim 1,wherein the second air/fuel mixture is a substantially rich air/fuelcondition.
 6. The method of claim 5, wherein the substantially richair/fuel mixture has an air/fuel ratio within a range that includes astoichiometric air/fuel ratio.
 7. The method of claim 6, wherein therange of substantially rich air/fuel condition is from about 95 percentto about 105 percent of the stoichiometric air/fuel ratio.
 8. The methodof claim 7, wherein the range of substantially rich air/fuel conditionis from about 90 percent to about 110 percent of the stoichiometricair/fuel ratio.
 9. The method of claim 1, wherein the first air/fuelcondition is a substantially lean air/fuel mixture.
 10. The method ofclaim 9, wherein the substantially lean air/fuel mixture has an air/fuelratio ranging from about 17:1 to about 20:1.
 11. The method of claim 1wherein the second air/fuel mixture is provided for a transientoperation of the engine.
 12. The method of claim 1, wherein theaftertreatment contains a catalyst that is operative to reduce thepollutant when an oxygen concentration in the exhaust gas is at or belowa predetermined level.
 13. The method of claim 1, further comprisingreducing a flow of recirculated exhaust gas to the engine whileproviding the second air/fuel mixture in the combustion chamber of theengine.
 14. The method of claim 1, wherein the step of providing thefirst air/fuel mixture further comprises the step of estimating anoxygen concentration in the combustion chamber and controlling a flow offuel in response to the estimated oxygen concentration.
 15. The methodof claim 14, wherein the step of estimating the oxygen concentration inthe combustion chamber further comprises the step of sensing the oxygenconcentration in the exhaust gas using at least one sensor.
 16. Themethod of claim 15, wherein the at least one sensor is a lambda sensor.17. The method of claim 1, wherein the compression ignition engine isdiesel fueled.
 18. The method of claim 1, wherein the compressionignition engine further comprises: a turbocharger, the turbocharger isdriven by a flow of exhaust gas from the engine, provides a flow ofcompressed air to the engine, and a controllable exhaust gasrecirculation valve that controls a recirculation flow of exhaust gasfrom the flow of exhaust gas from the engine to the turbocharger intothe flow of compressed air provided to the engine; and wherein the stepof providing a second air/fuel mixture to the combustion chamber of theengine further comprises closing the exhaust gas recirculation valve toincrease the flow of exhaust gas to the turbocharger.
 19. The method ofclaim 18 wherein: the turbocharger of the compression ignition enginehas a variable nozzle through which the exhaust gas flow passes to drivethe turbocharger, the variable nozzle causes the turbocharger toincrease flow and compression of air to the engine and in doing soincreases pressure of the exhaust gas flowing from the engine and thevariable nozzle causes the turbocharger to decrease flow and compressionof air to the engine and in doing so decreases pressure of the flow ofexhaust gas flowing from the engine; and wherein the step of providing asecond air/fuel mixture to the combustion chamber of the engine furthercomprises controlling the variable nozzle of the turbocharger toincrease flow of air to the engine.
 20. A method of operating acompression ignition engine to limit exhaust NO_(x) emissions duringfuel enriched operation to increase engine power comprising: Injectinghydrocarbon into an exhaust stream from the compression ignition engine;and treating the flow of exhaust gas with a non-urea aftertreatment toreduce a level of NO_(x) in the exhaust gas by combination with injectedhydrocarbon during a fuel enriched operation of the engine to invoke areal-time reduction of NO_(x) while increasing engine power.
 21. Amethod of operating a compression ignition engine to limit exhaustemissions comprising: operating the engine using a first fuel injectionpattern to inject fuel into a combustion chamber of the engine, whereinthe first fuel injection pattern comprises at least one pilot injectionand at least one main injection; receiving a request for an increasedpower output from the engine; operating the engine using a second fuelinjection pattern to inject fuel into the combustion chamber in responseto the request for the increased power output, wherein the second fuelinjection pattern consists of main only injection; and treating a flowof exhaust gas with a non-urea aftertreatment to reduce a level of atleast one pollutant in the exhaust gas during a fuel enriched operationof the engine to invoke a real-time reduction of the pollutant whileincreasing engine power.
 22. The method of claim 21, wherein thepollutant is soot.
 23. A method for use in an exhaust system of a dieselengine, the method comprising: providing a flow of an exhaust gas from acompression ignition engine, wherein the exhaust gas includes NOx;determining whether the compression ignition engine is being fueled by arich a rich air/fuel mixture; if the compression ignition engine isbeing fueled by a rich a rich air/fuel mixture, injecting hydrocarboninto the exhaust gas; and providing the exhaust gas injected with thehydrocarbon to a three-way catalyst system.
 24. The method of claim 23,further comprising determining whether the injected hydrocarbon hasreduced the NOx passing from the three-way catalyst system to apredetermined level.
 25. The method of claim 23, wherein the hydrocarbonis injected into the exhaust gas at a doser of the exhaust system. 26.The method of claim 23, wherein the hydrocarbon is injected into theexhaust gas at a burner of the exhaust system.
 27. The method of claim23, further comprising determining whether to elevate the amount ofhydrocarbon injected into the exhaust gas to accommodate for agingdeterioration of the three-way catalyst converter.